This invention relates to the manufacture of optical record media, and more specifically to a novel method of, and an apparatus for, making optical record media by utilizing radiant energy in the wavelength of ultraviolet. An example of record media to be fabricated by the invention is a digital audio disc which has a plastic baseplate or substrate having formed thereon a multiplicity of optically detectable marks of infinitesimal dimensions, such as depressions (pits) or projections, in a pattern representative of digitized information to be reproduce.
Digital audio discs, typically including what are known as compact discs (CDs) in common parlance, have won extensive commercial acceptance as record media of music and other information. Usually, the digital audio disc is a lamination of a transparent baseplate, a reflective layer and a protective overlay. The baseplate, molded from a transparent plastic such as polycarbonate, has a multiplicity of pits formed in one surface thereof in a multiturn spiral pattern. The reflective layer is formed on the pit-bearing surface of the substrate, as by vapor deposition of aluminum, and is covered by the protective overlay of a suitable plastic. The pit pattern is read by a laser beam impinging on the disk through the blank surface of the baseplate.
Conventionally, the baseplate with the pits has been formed by injection molding of polycarbonate or like plastic into dies defining a cavity in the shape of the disc-like substrate with the pattern of pits in one side thereof.
This conventional method is undesirable, first of all, because of large-size, high-power injection molding machines required for forming accurately the minute dimensions of the pits, even though the substrate itself to be molded is relatively small in size. The injection molding machines have also been undependable as to their capability of creating the pits to close dimensional tolerances required.
The dimensional accuracy of the pits is closely related to the inherent properties of the plastic employed as a material of the baseplate. Since the baseplate is semirigid, its warping affects the dimensional accuracy of the pits. The melting temperature of the plastic in use must also be taken into consideration in light of the warping of the resulting baseplate. Various proposals have so far been made for the solution of such problems arising from the conventional injection molding method. As far as the applicant is aware, none of them has gained general acceptance in the industry.
Like other forms of record media, optical discs are being constantly improved for greater storage capacities and greater storage densities (storage capacity per unit effective surface area or volume). The noted shortcomings of the conventional injection molding method would become all the more pronounced if it were used for the manufacture of flexible optical record media with a thickness of, say, 0.1 millimeter.